Pressure Broadening of the Inversion Spectrum of Ammonia: Part II - Disturbance of Thermal Equilibrium at Low Pressures

The absorption at the centre of the line (3, 3) of the centimetre wavelength inversion spectrum of ammonia gas has been measured at pressures between 1.5 mm. and 0.01 mm. Hg. The absorption coefficient is constant at the higher pressures, as would be expected for a single line whose width is determined solely by pressure broadening. At the lower pressures the absorption coefficient falls by an amount dependent on the energy density in the resonator; this is due to the disturbance of thermal equilibrium in the gas through the absorption of energy from the radio-frequency field. A theory of the effect is developed from which the thermal relaxation time can be calculated by comparison with the experimental values of the absorption coefficient. It is found that this time is approximately 1.7 5 times greater than the mean time between collisions which interrupt the absorption (determined from the line breadth constant). This indicates that not every such collision is effective in restoring thermal equilibrium.